The invention concerns a process for the production of a strip of hot rolled steel of very high strength, usable for shaping and particularly for stamping.
In the field of mechanical construction and more particularly of automobiles, the equipment particularly for safety, comfort and energy saving has given rise to research for lightening the weight whilst preserving the properties of durability and service of the stamped pieces. Fatigue strength, in particular, is an essential criterion because it defines the lifetime of these pieces. So as to improve this fatigue strength, one solution consists in the use of very high strength steels. There is effectively a linear relation between the limit of endurance and the mechanical strength. It is thus possible to use metal sheets with reduced thickness, which contributes to lightening the weight whilst keeping unchanged the durability and service. It is nevertheless necessary that the steel be adapted for stamping. However, in general, the properties of shaping decrease with the increase of mechanical resistance.
In the field of hot rolled steels, whose mechanical characteristics are obtained by controlled rolling in a wide strip mill, there exist particularly three types of hot rolled steels having high mechanical characteristics with an elastic limit comprised between 315 MPa and 700 MPa.
The HEL so-called high elastic limit steels, which are micro-alloyed steels having an elastic limit comprised between 315 MPa and 700 MPa, but a limited ability to be shaped, because in particular of an Re/Rm ratio comprised between 0.85 and 0.9.
The Dual-Phase steels, for their part, are steels of martensitic ferritic structure having remarkable shaping properties, but having a level of mechanical resistance not exceeding 600 MPa.
So-called HR steels which are carbon and manganese steels undergoing after rolling a rapid cooling associated with low temperature coiling, to give them a ferrite-bainite structure. These steels have shaping properties intermediate the HEL steels and the Dual-Phase steels. For example, HR steel 55 has a minimum resistance level of 540 MPa, and has a good ability to be stamped, with an Re/Rm ratio comprised between 0.75 and 0.8. Moreover, this steel is weldable and has an excellent ability to be given a shape of the raised flange type. Obtaining a steel of the HR 60 type requires either adding a micro-alloying element, for example niobium, which gives to this steel characteristics near those of an HEL steel, or increasing the carbon or manganese content of the HR 55 type steel, leading to a composition that can give rise to difficulty in the field of resistance welding.
The families of steels mentioned above thus have limits as to their mechanical characteristics and their behavior.
A metallurgical solution to improve the compromise between mechanical resistance and elongation, consists in using TRIP steels of residual ferrite-bainite-austenite structure. In this type of structure, the compromise between mechanical resistance and elongation is substantially improved by the presence, in the microstructure, of residual austenite. It is necessary in this case that the quantity of residual austenite be greater than 5%.
On the other hand, the presence of martensite in such a microstructure prevents improvement of stamping ability because of the presence of residual austenite.
A first possibility for obtaining TRIP steel is the use of steels of a composition of the Cxe2x80x94Mnxe2x80x94Si greater than 1% type. These compositions have the drawback of generating the formation of fayalite because of the presence of silicon.
Another possibility is the use of steels of the Cxe2x80x94Mnxe2x80x94Al composition type. This composition has insufficient residual austenite.
Obtaining residual austenite is possible only over a cladding temperature range comprised between 350xc2x0 C. and 400xc2x0 C., both for steels of the Cxe2x80x94Mnxe2x80x94Al TRIP type and for steels of the Cxe2x80x94Mnxe2x80x94Si TRIP type.
A coiling temperature below 350xc2x0 C. gives rise to the appearance of martensite, which particularly degrades the shapeability of the steels. Too high a coiling temperature leads to a purely ferrite-bainite structure without residual austenite, hence without improvement of the ability to be shaped. Thus, the presence of residual austenite must be greater than 5% to obtain an effect on the shapeability of the produced steels. Below this value, its influence is practically nothing.
Industrially, the coiling temperatures in the field mentioned above are particularly difficult to obtain. Thus, the range of coiling temperature between 350xc2x0 C. and 400xc2x0 C. corresponds to a region of instability of heat exchange between the steel strip and the cooling water, because of the breaking of the film of steam forming a screen between the hot metal and the cooling water. This phenomenon leads to an abrupt increase of the coefficient of heat exchange in the region in question, which gives rise, on the rolled steel strip, to a heterogeneity of the microstructure, which is prejudicial to the uniformity of the mechanical properties of the finished product. The need to use low coiling temperatures associated with the character of the TRIP compositions gives rise to difficulties in practice. There is thus sought an increase of the coiling temperature so as to enjoy greater ductility at high temperature.
The object of the invention is to perfect a process for the production of a steel strip of the TRIP type of very high strength, having good shaping properties.
The object of the invention relates to a process for the production of a hot rolled steel strip of very high strength, usable for shaping and particularly stamping, which is characterized in that the steel has the following weight composition:
carbon: 0.12-0.25%,
manganese: 1-2%,
aluminum: 0.03-2.5%,
silicon: 0.03-2%,
chromium: 0.04-2%,
phosphorus: 0.02-0.09%,
sulfur optionally up to 0.01%,
titanium up to 0.15%,
niobium up to 0.15%,
vanadium up to 0.15%, balance iron and residual impurities, is subjected to:
rolling at a temperature below 880xc2x0 C.,
a first short cooling, carried out over a time less than 10 seconds,
a second controlled cooling at a cooling speed V ref1 comprised between 20xc2x0 C./sec. and 150xc2x0 C./sec. as a function of the thickness of the rolled steel strip, the temperature at the end of the second cooling being below point Ar3 of the austenite-to-ferrite transformation, the temperature at the end of the second cooling being comprised between 700xc2x0 C. and 750xc2x0 C.,
holding at a temperature level associated with slow cooling, the speed of cooling being comprised between 3xc2x0 C./sec. and 20xc2x0 C./sec. to a temperature at the end of the level comprised between 700xc2x0 C. and 640xc2x0 C.,
a third cooling, also controlled, whose speed is comprised between 20xc2x0 C./sec. and 150xc2x0 C./sec., which cooling is according to the thickness of the metal strip; the temperature at the end of the third cooling being comprised between 350xc2x0 C. and 550xc2x0 C.
The other characteristics of the invention are:
the weight composition comprises less than 0.5% silicon,
the coolings are effected in the air,
the steel is hot rolled to obtain a hot rolled steel strip whose thickness is comprised between 1.4 mm and 6 mm.
The invention also relates to a hot rolled steel strip obtained by the process comprising in its composition, by weight:
carbon: 0.12-0.25%,
manganese: 1-2%,
aluminum: 0.03-2.5%,
silicon: 0.03-2%,
chromium: 0.04-2%,
phosphorus: 0.02-0.09%,
sulfur: optionally up to 0.01%,
titanium: up to 0.15%,
niobium: up to 0.15%,
vanadium: up to 0.15%, the balance iron and residual impurities.
The other characteristics of the invention are:
the hot rolled steel strip comprises in its weight composition less than 0.05% silicon,
the hot rolled strip has a thickness comprised between 1.4 mm and 6 mm.